The technical field of this invention is that of nondestructive materials characterization, in particular, quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy current sensors.
Characterization of bulk material condition includes measurement of changes in material state caused by fatigue damage, plastic deformation assessment, as well as assessment of residual stress, applied loads, and processing conditions such as, for example, heat treatment, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also may include measurements characterizing the material, such as alloy type, and material states, such as porosity and temperature.
Characterization of surface and near-surface conditions may include measurements of surface roughness, displacement or changes in relative position, coating thickness, and coating conditions. Each of these may involve detection of electromagnetic property changes associated with either microstructural and/or compositional changes, electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or, with single or multiple cracks, cracks or stress variations in magnitude, orientation, or distribution.
Conventional magnetometry, specifically, using eddy current sensors, involves excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This excitation produces a time-varying magnetic field at the same frequency, which, in turn, may be detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the self-impedance of the primary winding or the impedance between the primary and secondary windings. Traditionally, scanning of eddy current sensor across the material surface has been used to detect flaws.
For process and damage monitoring, including at elevated temperatures, it is desirable to detect and monitor material property changes during processing or to detect and monitor material damage during high-temperature service, as early as possible. This early detection allows process control and optimization in the case of process monitoring, and provides an early warning of potentially unsafe conditions in the case of the material damage monitoring. This is particularly critical for control of heat treatment processes such as steel tempering, alloy aging/precipitation hardening or other high-temperature processes as well as in-service monitoring of components operating at high temperatures.
As an example, steel tempering is typically done at temperatures between 350 and 1200° F., with lower temperatures producing higher strength steels. Aging is performed for a wide range of materials, including precipitation-hardening steels, nickel alloys, titanium alloys, aluminum alloys, magnesium alloys, etc. Aging temperature range depends on the class of materials. Some typical temperature ranges are: 900 to 1150° F. for precipitation-hardening steels, 800 to 1200° F. for titanium alloys, 250 to 675° F. for aluminum alloys, and 250 to 450° F. for magnesium alloys.
One limitation for the use of conventional eddy current sensors in high temperature applications are the calibrations requirements for meaningful property measurements. The calibration typically involves adjustment of the amplitude and phase (or the in-phase and out-of-phase) components to preselected conditions when the sensor is placed in well-defined proximity to the reference standard material having known properties. Since typical coils have many winding turns and the response is generally temperature sensitive, the reference standard needs to be at the same temperature as the test material and, for that matter, the sensor and associated instrumentation should also be at the nominal operating temperature under which the measurements will be performed. Otherwise, the elevated temperature measurements may be incorrect.